Generally, crude oil is refined in refineries to yield products, such as gasoline, diesel, and liquified petroleum gas (LPG), along with some other by-products. Such by-products include considerable amounts of heavy residues, which have to be upgraded to meet environmental legislations. The heavy residues can be used as feedstock in processes such as catalytic cracking, in which the heavy residues are contacted with a catalyst to obtain an additional yield of cracking products.
However, these heavy residues generally include contaminants, such as carbon residue, metal impurities, and basic nitrogen and sulphur compounds. These contaminants can adversely affect the catalytic cracking of the heavy residues. For example, the carbon residue may form carbonaceous deposits on the catalyst and thus reduce catalyst activity during processing of the feedstock. Further, certain metal impurities such as Nickel and Vanadium present in the heavy residues may accumulate on the catalyst and may lead to subsequent deactivation of the catalyst and undesirable hydrogen and coke formation.
Conventionally, during the catalytic cracking of heavy residues, the contaminants in the heavy residues are first separated from the feedstock to protect the catalyst from the contaminants. The separation may be achieved using various techniques, such as residue hydro-demetallation, residue desulphurization and metal passivation. However, these techniques require additional secondary processes, thus adding to the cost of processing the heavy residues. Further, techniques such as metal passivation require frequent changes in operating conditions of the catalytic cracking apparatus, which may render the operation of the apparatus ineffective in terms of cost.
In addition to the above mentioned techniques, techniques such as contaminant adsorption are also used conventionally. Such techniques employ a mixture of catalyst and adsorbent, in which the adsorbent removes the contaminants from the catalyst. Further, these techniques use physically separable mixtures of the catalyst and the adsorbent so that the adsorbent and the catalyst can be separated from each other by physical techniques, for example, under the effect of gravitational force (under fluidization conditions) or by using magnetic force, after the completion of the catalytic cracking. However, in such techniques, physical properties, for example, particle size and density, of the catalyst and the adsorbent have to be appropriately selected so that they may be separated easily. Hence, these techniques are limited by the physical properties of the adsorbent and the catalyst, and are usually economically unviable.